1. Field of the Invention
The present invention relates to a molten carbonate fuel cell and, more particularly, to a molten carbonate fuel cell in which an electrolyte body sandwiched between a pair of electrodes is improved.
2. Description of the Related Art
A basic structure of a molten carbonate fuel cell is shown in FIG. 1. An electrolyte body 3 retaining an electrolyte consisting of an alkali carbonate is sandwiched between an anode (fuel electrode) 1 and a cathode (air electrode) 2 which serve as a pair of electrodes. Two housings 4a and 4b abut against peripheral portions of both surfaces of the electrolyte body 3. The anode 1 and the cathode 2 are stored in the housings 4a and 4b, respectively. Corrugated collectors 5a and 5b are arranged in a space defined between the housing 4a and the anode 1 and a space defined between the housing 4b and the cathode 2, respectively. A supply port 6 for supplying fuel gas (H.sub.2 and CO.sub.2) to the anode 1 and an exhaust port 7 for exhausting exhaust gas (CO.sub.2 and H.sub.2 O) from the anode 1 are formed in the housing 4a in which the anode 1 is arranged. A supply port 8 for supplying an oxidant gas (air and CO.sub.2)to the cathode 2 and an exhaust port 9 for exhausting an exhaust gas (N.sub.2) from the cathode 2 are formed in the housing 4b in which the cathode 2 is arranged.
In the molten carbonate fuel cell shown in FIG. 1 an alkali carbonate mixture in the electrolyte body 3 is melted at a high temperature. The fuel gas (H.sub.2 and CO.sub.2) is supplied to the anode 1 through the supply port 6 of the housing 4a, while the oxidant gas (air and CO.sub.2) is supplied to the cathode 2 through the supply port 8 of the housing 4b. Thereby causing a reaction represented by formula (1) at the anode 1 and a reaction represented by formula(2) at the cathode 2: EQU H.sub.2 +CO.sub.3.sup.2- .fwdarw.H.sub.2 O+CO.sub.2 +2e.sup.-( 1) EQU 1/2O.sub.2 +CO.sub.2 +2e.sup.- .fwdarw.CO.sub.3.sup.2- ( 2)
The electrolyte body used in the molten carbonate fuel cell serves not only a medium for migration of carbonate ions (CO.sub.3.sup.2-) but also a gas permeation barrier layer for inhibiting direct mixture (gas crossover) of reaction gases between the anode and the cathode. In order to perform these functions, the electrolyte must be sufficiently retained in the electrolyte body. An outflow of the electrolyte (electrolyte loss) increases an internal resistance and occurrence of a gas crossover.
The electrolyte body is basically obtained by a matrix method in which a porous body is formed by a ceramic filler, and the porous body is impregnated with an electrolyte consisting of an alkali carbonate mixture containing at least two carbonates selected from the group consisting of Li.sub.2 CO.sub.3, K.sub.2 CO.sub.3, and Na.sub.2 CO.sub.3.
The filler consists of particles (to be referred to as retaining particles hereinafter) having a particle size of, e.g., 1 .mu.m or less and a function of retaining the electrolyte, and particles (to be referred to as reinforcing particles hereinafter) having a particle size of, e.g., 10 .mu.m or more and a function of reinforcing the porous body. The function of retaining the electrolyte indicates retention of the electrolyte as a fluid and prevention of outflow thereof during a high-temperature operation. The function of reinforcing the porous body indicates prevention of cracks and collapse of the porous body upon a rise and a fall in temperature.
Conventionally, the retaining particles constituting the filler are LiAlO.sub.2 particles having a specific surface area of 5 m.sup.2 /g to 25 m.sup.2 /g. The reinforcing particles are LiAlO.sub.2 particles homogeneous to the retaining particles.
A compound (e.g., LiAlO.sub.2) relatively stable in a molten carbonate is used as the filler. An electrolyte body comprising a porous body containing the LiAlO.sub.2 retaining and reinforcing particles as the filler poses the following problem. When the porous body is kept together with a highly corrosive molten carbonate for a long period of time, the retaining particles are absorbed in the reinforcing particles having a larger particle size than that of the retaining particles. The retaining particles thus disappear. As a result, the pore size of the porous body as a skeleton for retaining the electrolyte is increased. The retaining function of the electrolyte of the electrolyte body is degraded to cause outflow of the electrolyte to result in an electrolyte loss and a gas crossover accompanying local dissipation of the electrolyte. Cell performance properties are then greatly degraded.